Free Piston Expander-linear Generator Research Articles

Free piston expander-linear generator used for organic Rankine cycle waste heat recovery system

This study presents an experimental investigation of a free piston expander-linear generator (FPE-LG) used for organic Rankine cycle (ORC) waste heat recovery system. A FPE-LG test rig using compressed air as working fluid is established. The motion characteristics, dynamic characteristics and the indicated efficiency of FPE-LG are analyzed. The motion characteristics and power output performance for different valve timings are studied. The degree of symmetry is defined to evaluate the asymmetry motion characteristics of the free piston assembly. The coefficient of cycle-to-cycle variation (COV) is presented to evaluate the cycle-to-cycle variation characteristics of the FPE-LG. Experimental results show that the free piston assembly displacement profile is similar to a sinusoidal wave and the free piston assembly can operate at high and relatively constant speed at the middle portion of the stroke. The maximum power output of 19W can be achieved when the intake pressure is 2.0bar and the operation frequency is 2.5Hz. The valve timing and intake pressure demonstrate a significant influence on the asymmetric motion and the power output performance of the FPE-LG. The indicated efficiency of the FPE (left cylinder) decreases with the increase in the intake pressure. The maximum indicated efficiency reaches 92.8% when the intake pressure is 1.4bar and the operation frequency is 2.0Hz. The indicated efficiency firstly increases and then decreases with the increase in the operation frequency. The COV of the FPE-LG decreases with increasing the intake pressure. The motion stability of FPE-LG improves with the increase in the intake pressure. Valve timing and valve train should be optimized in the near future.

Preliminary Development of a Free Piston Expander–Linear Generator for Small-Scale Organic Rankine Cycle (ORC) Waste Heat Recovery System

A novel free piston expander-linear generator (FPE-LG) integrated unit was proposed to recover waste heat efficiently from vehicle engine. This integrated unit can be used in a small-scale Organic Rankine Cycle (ORC) system and can directly convert the thermodynamic energy of working fluid into electric energy. The conceptual design of the free piston expander (FPE) was introduced and discussed. A cam plate and the corresponding valve train were used to control the inlet and outlet valve timing of the FPE. The working principle of the FPE-LG was proven to be feasible using an air test rig. The indicated efficiency of the FPE was obtained from the p–V indicator diagram. The dynamic characteristics of the in-cylinder flow field during the intake and exhaust processes of the FPE were analyzed based on Fluent software and 3D numerical simulation models using a computation fluid dynamics method. Results show that the indicated efficiency of the FPE can reach 66.2% and the maximal electric power output of the FPE-LG can reach 22.7 W when the working frequency is 3 Hz and intake pressure is 0.2 MPa. Two large-scale vortices are formed during the intake process because of the non-uniform distribution of velocity and pressure. The vortex flow will convert pressure energy and kinetic energy into thermodynamic energy for the working fluid, which weakens the power capacity of the working fluid.